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Structures Related to the Emplacement of Shallow-Level Intrusions David Westerman, Sergio Rocchi, Christoph Breitkreuz, Carl

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Structures Related to the Emplacement of Shallow-Level Intrusions David Westerman, Sergio Rocchi, Christoph Breitkreuz, Carl Structures related to the emplacement of shallow-level intrusions David Westerman, Sergio Rocchi, Christoph Breitkreuz, Carl Stevenson, Penelope Wilson 1. Norwich University, VT, USA 2. Università di Pisa, Italy 3. TU Bergakademie, Freiberg, Germany 4. University of Birmigham, UK 5. Kingston University, London, UK Abstract A systematic view of the vast nomenclature used to describe the structures of shallow level intrusions is presented here. Structures are organised in four main groups, according to logical breaks in the timing of magma emplacement, independent of the scales of features: (1) Intrusion-related structures, formed as the magma is making space and then develops into its intrusion shape; (2) Magmatic flow-related structures, developed as magma moves with suspended crystals that are free to rotate; (3) Solid-state, flow-related structures that formed in portions of the intrusions affected by continuing flow of nearby magma, therefore considered to have a syn-magmatic, non-tectonic origin; (4) Thermal and fragmental structures, related to creation of space and impact on host materials. This scheme appears as a rational organisation, helpful in describing and interpreting the large variety of structures observed in shallow-level intrusions. Keywords: Magma intrusion, Magma flow, Syn-magmatic deformation, Fragmentation, Geo-logic. 1. Introduction This overview of the fabrics and structures characterizing shallow-level intrusions is focused on tabular intrusions recognized as dykes, sills and laccoliths, and related types of bodies. At the grossest scale, shallow-level intrusive bodies are classified based on their aspect ratio and relationship to pre-existing structures in their host, and on their coherent versus clastic texture. The simplest subdivision is generally to separate dykes, with their subparallel contacts cross cutting the host fabric, from sills (s.l.). 1 The latter group is further subdivided into sills (s.s.), laccoliths, and a host of more esoteric forms (Rocchi and Breitkreuz this volume). Sills are, for the most part, defined as tabular igneous intrusions that are concordant with planar structures in the surrounding host rocks (bedding, unconformities, or cleavage) and display large length:thickness ratios. Laccoliths and lopoliths are also concordant intrusions with a conveX-upwards upper contact or a planar to concave-downwards lower contact, respectively. Laccoliths are distinguished from sills by the non-linear, inelastic, large-scale deflection of roof host rocks that accompanies their emplacement, and by their generally smaller length:thickness ratios (Corry 1988). Because bodies such as dykes, sills and laccoliths develop by magma propagating along a planar surface, either by brittle or non-brittle failure, it is useful to acknowledge the time frame within which we are concerned. This chapter focuses on the interval between the arrival of the initial magma at its propagating fracture (a time before the igneous body was there at all) and the cessation of magmatic activity with development of younger fractures due to cooling (a time after it was there completely). This broad scope reflects the transitional role that SLI play as an interface between the hidden kingdom of Pluto below and the fiery realm of Vulcan above. Although nearly all of the features discussed here have been previously recognized and described, including in some of the earliest writings on geology, our organization and coverage of this material can provide a framework to visualize the formation of SLI through time and space. For the most part, SLI share the characteristic of having distinctive chilled margins, a population of phenocrysts, and generally aphanitic textures in the matrix. The simplest interpretation is that generally euhedral phenocrysts formed prior to arrival at the final emplacement level (Mock et al. 2003), and that solidification of the melt phase was rapid. EXceptions include thick mafic sills that are characterized by phaneritic textures, often with evidence of layering attributed to in-situ gravity settling (e.g.: 350-450 m-thick Basement Sill, Antarctica) (Bédard et al. 2007) and, more rarely, moderately thick (ca. 50 m) rhyolitic sills (Orth and McPhie 2003). The scales of these features range from structures consisting of sets of intrusive bodies, to shapes and sizes of individual bodies, to scales related to maps, outcrops, hand specimens, thin sections and smaller. Also included are invisible properties such as anisotropy of magnetic susceptibility and chemical zonation. The value of understanding the physical characteristics of these shallow intrusions lies in large part with the information they can provide about their modes of formation. Often of greatest interest is the use of these characteristics to unravel directions and styles of magma flow so as to understand the growth and development of these bodies. 2 Major works that bring the understanding of dykes to the forefront include papers collected in proceedings from the International Dyke Conference starting as far back as 1985 (Halls and Fahrig 1987). Papers of this volume, along with volumes resulting from previous LASI conferences since the first in 2002, provide a useful collection of modern work detailing the physical geology of laccoliths, dykes and sills (Breitkreuz and Petford 2004; Rocchi et al. 2010; Thomson and Petford 2008). Principally, formation of all fabrics and structures featured here depends on fundamental physical parameters of the emplacing magma, such as temperature, driving pressure, viscosity, volume, ascent/emplacement rate and emplacement level, as well as yield strength and porosity of the host. Many of these parameters have been eXplored in other chapters of this volume. Understanding of structures in sub-horizontal tabular intrusions starts with the generalized characteristics that have been addressed by examining expansive databases of sills and laccoliths as reported in the literature. The database of the first encyclopedic work (Corry 1988) recorded 900 laccoliths globally, 600 of which were from North America. The author eXtrapolated this number to infer that there were actually 5,000 to 10,000 laccoliths globally. This database has been eXpanded and the diameter and thickness values have been examined, discovering the power law relation between those values (Cruden and McCaffrey 2001; McCaffrey and Petford 1997). Subsequently, an S- shaped power law matching well with plutons of all scales has been proposed (Cruden and McCaffrey 2002), and further augmented and discussed by Cruden et al. this volume. The resulting models show consistent geometric relationships of these parameters. The multi-layered laccoliths preserved on Elba Island have provided a number of eXamples that support published models (Cruden and McCaffrey 2002; Cruden and McCaffrey 2001), but also yield further understanding about the emplacement and inflation processes within such compleXes (Rocchi et al. 2002) and the relationships of their geometries to those of assembled plutons (Rocchi et al. 2010). The range of features associated with formation of tabular SLI is very broad. During emplacement and solidification, fabrics and structures develop in different locations within the bodies (interior vs. interface), from different driving forces (magmatic flow, solid-state flow, static gravity and thermal), and at various scales (from whole intrusion scale to microscale). SLI are characterized first by generally having large aspect ratios with overall tabular forms, and second by emplacement into the coolest portion of the crust. These conditions lead to the relatively short “lifetimes” of these bodies between onset of emplacement and complete solidification. Compared to more deep-seated plutons, SLI end up dominated by fabrics and structures directly associated with emplacement since their typical quick crystallization limit the time available to fractionate and develop 3 features associated with in-situ crystallization. Thus, well-preserved fabrics in SLI are helpful to better understand the early history of deeper-level plutonic compleXes in which many teXtures are erased by prolonged internal movements and crystallization. 2. Structural Subdivisions Scheme As with many subdivision schemes, the one presented here is fraught with the complexities of miXing description and genesis, together with space and time as they relate to the progressive growth of SLI. We have chosen to first eXamine structures associated with the openings and modes of magma intrusion-emplacement to build SLI (see overview in Table 1). Our second focus is on structures and fabrics directly related to the growth of the bodies as magma is moving and suspended particles are free to rotate, i.e. the magmatic flow stage (overview in Table 2). NeXt, we review those fabrics and structures that form after crystals can no longer rotate, but rather respond by solid-state deformation driven by ongoing magmatic processes that are exclusive of external tectonic events (Table 3). Finally, we look at features developing in response to thermal gradients and gravitational forces, as well as marginal structures that help to provide space for magma and that result from fragmentation processes involving igneous and host materials (Table 4). 2.1. Intrusion-related structures At the onset of opening to build a new SLI, the width of the opening, by definition, starts at nothing and eventually reaches a maximum. Of course subsequent deflation is possible, as is back flow in dykes following
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